Kenwood R-1000 popping audio on power-up

[Opcom]

I use my Kenwood R-1000 for the 2nd receiver. Nice portable unit too.

The audio began to make a single pop when turned on, then after a while the audio was gone. I replaced the audio output IC with an NTE due to no audio output from it, and this seems perfectly fine. The speaker has been blown by the problem too.

So, after replacing the audio output IC, it works well, but whenever it is turned on, the audio output pin voltage, normally about 6V, goes into a + and - oscillation, looks like like rail to rail.

The AC part, the audio, passes through the output coupling cap and drives the speaker banging it back and forth.

It does this about 4 to 6 times then stops and is perfectly OK for listening. The pulses are about 10ms wide and spaced about 200ms. This looks like some kind of motor-boating, except it stops after making the annoyance.

Has anyone else had this audio popping or motor-boating issue on the TS430?

The schematic copy is attached in case anyone wants to take a stab at suggestions. It's not the best but OK.

[WQ9E]

Patrick,

Are you sure it isn't on the input line to the audio power IC?  Check here with a scope to be sure.

Is the NTE device an exact replacement or does it perhaps have higher gain?  Leaky C187 perhaps?

I note that the AM narrow/wide switch is tied into the audio amp via Q47, does this have any impact on the symptoms?

I checked my Kenwood service document for the R-1000 and it doesn't mention this problem.  The only notes are for updates to the muting, AM AGC time constant, and attenuator steps.  My R-1000 doesn't have any thumps or other issues at startup.

[Opcom]

The Q47 looks like tone control that makes bass response 'normal' for AM wide as pin 5 is the inverting input.
The bandwidth or mode AM/SSB does not make any difference.

The trouble is trying to work on this with the board hooked up. The IC is at the edge so the pins can e observed.

I have to check the input, that's a good point. But it happens even if the volume is at zero, so nothing should be there.

The IC power comes right from the unregulated DC through the 5.6R resistor. I checked the resistor in the past to be OK.

I might want to replace the output coupling cap and C186 the power supply decoupling cap. I put a 330 Ohm resistor instead of the speaker and didn't find the slightest trace of DC leakage. With 330 Ohms there would have been a few mV at least? The popping wave form looks the same on each side of the output coupling cap except the DC shift. It may be time to check every component around there.

. It had started to have this issue slowly over time, and I guess it happened so slowly I did not notice it properly. It was in a high noise environment of a military truck shelter so it may not have been so noticeable.

Then the amp IC blew out. The speaker is open, I assume due to that but how can I say what really happened there.

There's also an issue with the mode switches that they do not seem to turn on the switching diodes consistently. I have cleaned them. I think that is secondary to this issue. Since I've opened the radio the frequency is shifted down some 300KC. Odd as it is a non digital tuner and uses a standard frequency counter method, not a CPU synthesizer. The counter is on frequency of stations. This makes me wonder if the power supply needs checked more closely as well. Having time to think on it.

I am pretty sure the NTE is an exact replacement, there is no other substitute.

[w3jn]

Replace C187 and C191.  Either one, the other, or both is/are bad.  I'd also suspect C183, C184, and C158.

If C191 shorts or is extremely leaky it's going to shove DC right thru the speaker coil.  That'll also upset the IC's internal bias and could very well cause your oscillations.

That receiver is getting to the place where the electrolytics are all suspect.  I'd roll up my sleeves and go to town wholesale on 'em.

[Opcom]

C191 the output DC blocking cap -drive the speaker -was low on capacity.
Replaced it and C186 the power supply decoupling cap.
Main cause: The resistor R175, a 5.6 Ohm 1/4W unit that goes between the unregulated DC and the audio amp (and C186) was intermittent.
It would check good, then not. Probably ruined when the old IC shorted. Not flameproof yet looked nice.
After replacing it, the popping complaint went away. The other caps checked perfectly fine, but i may replace them later once I restock caps. For some off reason I don't have very many kinds of caps used for low voltage solid state stuff.

One thing I found in this set is the plug-in connectors on the boards can just 'fall out'. The solder is loosening around the pins and this can not be seen from the copper side of the board, so I have to go hit them all each pin, and check very carefully. The old phenolic type board won't stand much heat either. The 'vintage' is 1979 so the set is 36 years old.

For some reason the set is now grossly insensitive. Must be losing my touch due to inactivity. Maybe I left a tiny little plug off or something or it is another plug with bad solder. But I'm happy this weird issue is solved. I really like repairing things and can see that if I had a job doing it then I'd be very efficient at it again.